Sažetak
We report on specific heat measurements of the quasi-one-dimensional organic salt (TMTSF)_2AsF_6 in its spin density wave state between 75 mK and 7 K. Similarly to (TMTSF)_2PF_6, we find discontinuities in the lattice contribution at 1.9 K and 3.5 K ascribed to sub-spin density wave phases. Time-dependent effects due to dynamics of low-energy excitations in metastable states occur only below 0.2 K which yields an activation energy for the equilibrium energy relaxation process of 0.34 K, 4-5 times smaller than found for (TMTSF)_2PF_6. Finally the reduction of the low-energy excitations contribution to the specific heat in comparison to PF_6 reveals an intermediate cubic-like regime between 0.25 and 0.5 K that we tentatively describe as the phason contribution of the incommensurate spin density wave modulation.